Solid state membranes formed from oxygen ion conducting materials are beginning to show promise for use in commercial processes for separating oxygen from oxygen-containing streams. Envisioned applications range from small scale oxygen pumps for medical use to large scale integrated gasification combined cycle (IGCC) plants. This technology encompasses two distinctly different membrane materials, solid electrolytes and mixed conductors. Membranes formed from mixed conductors are preferred over solid electrolytes in processes for separating oxygen from oxygen-containing gaseous mixtures because mixed conductors conduct both oxygen ions and electrons and can be operated without external circuitry such as electrodes, interconnects and power-supplies. In contrast, solid electrolytes conduct only oxygen ions.
Membranes formed from solid electrolytes and mixed conducting oxides are oxygen selective and can transport oxygen ions through dynamically formed oxygen anion vacancies in the solid lattice when operated at temperatures typically above about 700.degree. C. Examples of solid electrolytes include yttria-stabilized zirconia (YSZ) and bismuth oxide. Examples of mixed conductors include titania-doped YSZ, praseodymia-modified YSZ, and, more importantly, various mixed metal oxides some of which possess the Perovskite structure. Japanese Patent Application No. 61-21717 discloses membranes formed from multicomponent metallic oxides having the Perovskite structure represented by the formula La.sub.1-x Sr.sub.x Co.sub.1-y Fe.sub.y O.sub.3-d wherein x ranges from 0.1 to 1.0, y ranges from 0.05 to 1.0 and d ranges from 0.5 to 0.
Membranes formed from mixed conducting oxides which are operated at elevated temperatures can be used to selectively separate oxygen from an oxygen-containing gaseous mixture when a difference in oxygen partial pressures exist on opposite sides of the membrane. Oxygen transport occurs as molecular oxygen is dissociated into oxygen ions which ions migrate to the low pressure side of the membrane where the ions recombine to form oxygen molecules, and electrons migrate through the membrane in a direction opposite the oxygen ions to conserve charge. The rate at which oxygen permeates through the membrane is mainly controlled by two factors, the diffusion rate within the membrane and the kinetic rate of interfacial oxygen exchange; i.e., the rate at which oxygen molecules in the feed gas are converted to mobile oxygen ions at the surface of the feed side of the membrane and back again to oxygen molecules on the permeate side of the membrane.
Membranes formed from mixed conducting oxides offer substantially superior oxygen selectivity than polymeric membranes. However, the value of such improved selectivity must be weighed against the higher costs associated with building and operating plants employing membranes formed from mixed conducting oxides which plants require heat exchangers, high temperature seals and other costly equipment. Typical prior art membranes formed from mixed conducting oxides do not exhibit sufficient oxygen permeance (defined as a ratio of permeability to thickness) to justify their use in commercial gas separation applications.
Oxygen permeance through solid state membranes is known to increase proportionally with decreasing membrane thickness until the membrane thickness approaches about 0.5 mm. Many steps are involved in converting molecular oxygen to mobile oxygen ions, which oxygen ions are transported through the solid state membrane and converted back to molecular oxygen on the opposite side of the membrane. Each of these steps contributes to impede transfer of oxygen through thin solid state membranes, particularly those having a thickness less than about 0.5 mm.
Teraoka and coworkers, J. Ceram. Soc. Japan. International Ed, Vol 97, pp 458-462, (1989) and J. Ceram. Soc. Japan. International Ed, Vol 97, pp 523-529, (1989) describe solid state gas separation membranes formed by depositing a dense, nonporous mixed conducting oxide layer, referred to as "the dense layer", onto a porous mixed conducting support. The relatively thick porous mixed conducting support provides mechanical stability for the thin, relatively fragile dense, nonporous mixed conducting layer. Structural failures due to thermo-mechanical stresses experienced by the membranes during fabrication and use were substantially minimized due to the chemical compatibility of the respective membrane layers. Based upon considerations limited to dense layer thickness, Teraoka and coworkers expected the oxygen flux to increase by a factor of 10 for a membrane having a mixed conducting porous layer and a thin mixed conducting dense layer compared to a standard single layered dense, sintered mixed conducting disc. However, they obtained an increase of less than a factor of two.
Researchers are continuing their search for thin, supported solid state ionically conductive membranes which exhibit superior oxygen flux without sacrificing mechanical and physical compatibility of the composite membrane.